(1) Field of the Invention
The present invention relates to a method and to a system for assisting piloting in order to avoid an obstacle with a rotorcraft, and it also relates to a rotorcraft implementing the method.
(2) Description of Related Art
A rotorcraft has a rotary wing provided with a rotor. The rotor contributes at least in part to providing the aircraft with lift, and possibly to providing the aircraft with propulsion.
The invention relates to the general technical field of assisting in the piloting of rotorcraft, and in particular to the fields of alerts for avoiding obstacles. The present invention relates more particularly to a method of issuing an alert in order to enable a rotary wing aircraft to avoid an obstacle, and also to a device implementing the method, and to an aircraft having such a device.
The term “obstacle” is used broadly. The term obstacle applies equally well to natural obstacles, i.e. the terrain being overflown, and to obstacles that are not natural, e.g. such as a building or a cable. The term “obstacle” in isolation thus refers to any item that might impact against the aircraft.
Such systems for assisting piloting by issuing an alert to avoid an obstacle are known as terrain avoidance warning systems (TAWS).
Such TAWS piloting assistance systems serve to indicate dangerous obstacles ahead of the aircraft on its path, while they are being approached. Such TAWS piloting assistance systems include in particular an obstacle avoidance function known as forward-looking terrain avoidance (FLTA). Using the FLTA avoidance function, a TAWS piloting assistance system may seek, for example, to prevent an aircraft hitting the ground without causing the pilot at the controls to lose control, in particular as a result of a navigation error or indeed by misjudging the height of relief being overflown or of non-natural obstacles, if any.
Another system is known as a ground proximity warning system (GPWS), serving to alert the pilot of the aircraft that the ground is getting closer.
All of these systems have been designed mainly for airplanes and they are not necessarily suitable or satisfactory for rotorcraft. A rotorcraft is capable of performing numerous different types of flight, in comparison with a fixed wing aircraft such as an airplane. Only cruising flights of a rotorcraft are genuinely comparable with airplane flights. An aircraft can also hover, or fly only vertically, or only sideways. Furthermore, a rotorcraft can fly for long periods of time very close to the ground and obstacles that are not natural.
During such flights, the parameters that are taken into consideration and the alerts that are issued by a terrain avoidance system designed for an airplane are not appropriate, and possibly even undesirable, and they can constitute a nuisance for the pilot.
Furthermore, the piloting controls of a rotary wing aircraft are different from those of an airplane, so performing an avoidance maneuver requires specific piloting actions that are different from those applicable to airplanes.
Given this observation, systems for assisting piloting by using an alert in order to avoid terrain have been adapted for rotorcraft and they are known as helicopter terrain avoidance warning systems (HTAWS).
An HTAWS piloting assistance system can act for example to determine an avoidance path for the aircraft with the help of flight data. On each calculation iteration performed by the system, the avoidance path conventionally includes a segment extending from the aircraft along the current speed vector of the aircraft.
The term “current speed vector” refers to the speed vector of the aircraft at the moment the calculation iteration is performed.
Furthermore, such an HTAWS piloting assistance system can evaluate whether the avoidance path runs the risk of leading the aircraft towards an obstacle. If so, a so-called “terrain” alarm is triggered.
A rotorcraft, and in particular a helicopter, is provided with a lift rotor. On board a helicopter, a lift rotor contributes at least in part to propelling the aircraft. Such a rotor is sometimes referred to as a “main” rotor.
Modifying the cyclic pitch of the blades of the main rotor tends to tilt the speed vector of the rotorcraft.
In order to control a rotorcraft, a pilot can thus operate a cyclic pitch stick in order to act on the cyclic pitch of the blades or the main rotor.
Furthermore, a pilot can operate a collective pitch lever in order to act on the collective pitch of the blades of the main rotor.
A modification to the collective pitch causes the rotorcraft to move down or up.
In order to avoid an obstacle such as a mountain, for example, a pilot can make use of a resource by operating the cyclic pitch stick. The resource that is available is physically limited by the load factor that the aircraft can accept.
The resource thus leads to the current speed vector of the aircraft tilting, and consequently to the avoidance path prepared by the piloting assistance system tilting.
Specifically, the avoidance path is modified as a result of the resource and conventionally it tends to move suddenly upwards. This avoidance path might then reach a point that is situated above the obstacle.
Under such circumstances, certain THAWS piloting assistance systems consider that the aircraft is out of danger relative to the obstacle that is to be avoided. Nevertheless, the aircraft is still in danger so long as the aircraft is not actually above the obstacle to be avoided and independently of its current speed vector. Under such conditions, the piloting assistance system runs the risk of not generating an alert even though the aircraft is not entirely safe.
The object of the invention is to deliver a “terrain” alarm representative of the energy balance of the rotorcraft in order to avoid such a situation.
Document FR 3 008 530 describes an HTAWS type piloting assistance system that calculates a sheet of avoidance paths. Each avoidance path includes a preliminary stage during which the pilot has not yet reacted, and thus during which the avoidance path is purely an extension of the current path and thus of the current speed vector.
Document FR 2 932 919 describes a terrain alert system that takes account of the instantaneous maneuverability of the aircraft, and in particular its instantaneous mass and vertical acceleration. An avoidance path is then prepared and is constituted by a substantially rectilinear proximal segment representative of a transfer time together with at least one distal segment of conical profile. The proximal segment then extends the path being followed, i.e. the direction of the current speed vector of the aircraft.
Document US 2002/0030610 describes a method that describes an angle of the current speed vector relative to the horizontal in order to calculate an alarm.
Document US 2002/0126040 proposes using an extension to the current path in order to present different levels of terrain in different colors on a screen.
Document U.S. Pat. No. 8,249,799 proposes using an extension of a current path in order to determine whether an obstacle is dangerous.
Document U.S. Pat. No. 7,941,250 is remote from the field of the invention. This document is mentioned solely by way of information and it relates to calculating a predicted vertical speed.
Document U.S. Pat. No. 5,781,126 is also known.